Plate Tectonics

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Theory of Plate Tectonics
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Plate Tectonics

• According to the theory of plate tectonics,
  Earths crust and part of the upper mantle are
  broken into sections.

• These sections, called plates, move on a
  plasticlike layer of the mantle.

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Theory of Plate Tectonics
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Composition of Earths Plates

• Plates are made of the crust and a part of the
  upper mantle.

• These two parts combined are the lithosphere (LIH
  thuh sfihr).

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Theory of Plate Tectonics
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Composition of Earths Plates

• The plasticlike layer below the lithosphere is
  called the asthenosphere (as THE nuh sfihr).

• The rigid plates of the lithosphere float and
  move around on the asthenosphere.

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Theory of Plate Tectonics
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Plate Boundaries
When plates move, they can interact in several
ways, the result of their movement is seen at the
plate boundaries.
Plates can Converge, or collide or pull apart or
slide alongside one another
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Theory of Plate Tectonics
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Plate Boundaries

• Any plate Movement means that changes will
  happen at other boundaries.

• What is happening to the Atlantic Ocean floor
  between the North American and African Plates?

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• There are 3 types
• of plate boundaries
• Each boundary will have unique features.

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Theory of Plate Tectonics
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Plates Moving Apart
1. Divergent boundary The boundary between two
plates that are moving apart.

• 2. Mid Ocean Ridge
• New sea floor being made
• Underwater Volcanic mtns.

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• Other features found at the Mid-Ocean Ridge
• Mountains
• Valleys
• Earthquakes

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Mid-Oceanic Ridge (new seafloor being made)
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Theory of Plate Tectonics
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Plates Moving Apart
An Example
The Atlantic Ocean, the N. American Plate is
moving away from the Eurasian the African
Plates.
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Another type of Divergent boundary is the Great
African Rift Valley

• Features found at a Rift Valley
• Fault-Block Mountains
• Normal Faults created by a tension force

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Theory of Plate Tectonics
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Normal Faults and Rift Valleys

• When rocks break and move along surfaces, a fault
  forms.

• Faults interrupt rock layers by moving them out
  of place.

• Entire mountain ranges can form in the process,
  called fault-block mountains

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Theory of Plate Tectonics
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Plates Moving Together
2. Convergent boundary occurs where two plates
move together.
As new crust is added in one place, it disappears
below the surface at another Place. The
disappearance of crust can occur when seafloor
cools, becomes denser, and sinks.
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Continental - continental convergence
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Theory of Plate Tectonics
Where Two Continental Plates Collide They form

• Mountain ranges - Because these plates are less
  dense than the material in the asthenosphere,
  when they collide they crumple up.

• Earthquakes are common at these convergent
  boundaries.

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Plates Moving Together
Ocean - continental convergence
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Theory of Plate Tectonics
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Plates Moving Together
When an ocean plate converges with a less dense
continental plate, the denser oceanic plate sinks
under the continental plate.

• Subduction zone The area where an oceanic plate
  subducts, or goes down, into the mantle.

• Features found at a S.Z.
• Volcanic mtns
• Earthquakes

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Theory of Plate Tectonics
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Plates Moving Together

• Some volcanoes form above subduction zones.

• Deep-sea trench type of convergent boundary
  created where one plate bends and sinks beneath
  the other.
• Old Sea Floor being destroyed

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Plates Moving Together
Ocean - continental convergence
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Theory of Plate Tectonics
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Plates Moving Together

• High temperatures cause rock to melt around the
  subducting slab as it goes under the other plate.

• The newly formed magma is forced upward along
  these plate boundaries, forming volcanoes.

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Theory of Plate Tectonics
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Where Plates Collide

• A subduction zone also can form where two oceanic
  plates converge.

• In this case, the colder, older, denser oceanic
  plate bends and sinks down into the mantle.

• Usually, no subduction occurs when two
  continental plates collide. Folded mtns form.

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Plates Moving Together
Ocean - continental convergence
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Theory of Plate Tectonics
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Where Plates Slide Past Each Other
3. Transform boundary Occur where two plates
slide past one another

• They move in opposite directions or in the same
  direction at different rates.

• Features
• Earthquakes

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Theory of Plate Tectonics
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Where Plates Slide Past Each Other

• When one plate slips past another suddenly,
  earthquakes occur.

• The San Andreas Fault is part of a transform
  plate boundary. It has been the site of many E.
  Q.

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Theory of Plate Tectonics
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Causes of Plate Tectonics

• Convection current The cycle of heating, rising,
  cooling, and sinking.

• This process occurs in the mantle, it is thought
  to be the force behind plate tectonics.

• Differences in density cause hot, plasticlike
  rock to be forced upward toward the surface.

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Theory of Plate Tectonics
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Moving Mantle Material

• In one hypothesis, convection currents occur
  throughout the mantle.

• Such convection currents (see arrows) are the
  driving force of plate tectonics.

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Theory of Plate Tectonics
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Features Caused by Plate Tectonics

• As plates move, they interact.

• The interaction of plates produces forces that
  build mountains, create ocean basins, and cause
  volcanoes.

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Theory of Plate Tectonics
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Features Caused by Plate Tectonics

• When rocks in Earths crust break and move,
  energy is released in the form of seismic waves.

• Humans feel this release as earthquakes.

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Theory of Plate Tectonics
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Normal Faults and Rift Valleys

• Rift valleys and mid-ocean ridges can form where
  Earths crust separates.

• Examples of rift valleys are the Great Rift
  Valley in Africa, and the valleys that occur in
  the middle of mid-ocean ridges.

Click image to view movie.
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Theory of Plate Tectonics
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Mountains and Volcanoes

• As continental plates collide, the forces that
  are generated cause massive folding and faulting
  of rock layers into mountain ranges such as the
  Himalaya.

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Theory of Plate Tectonics
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Mountains and Volcanoes

• The type of faulting produced is generally
  reverse faulting.

• When two oceanic plates converge, the denser
  plate is forced beneath the other plate.

• Curved chains of volcanic islands called island
  arcs form above the sinking plate.

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Theory of Plate Tectonics
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Mountains and Volcanoes

• If an oceanic plate converges with a continental
  plate, the denser oceanic plate slides under the
  continental plate.

• Folding and faulting at the continental plate
  margin can thicken the continental crust to
  produce mountain ranges.

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Theory of Plate Tectonics
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Strike-Slip Faults

• In a strike-slip fault, rocks on opposite sides
  of the fault move in opposite directions, or in
  the same direction at different rates.

• When plates move suddenly, vibrations are
  generated inside Earth that are felt as an
  earthquake.